Single crystal and multiple crystal shaped articles are used in a host of applications. For example, windows transparent to radiation in the range from microwave through infrared and ultraviolet radiation are used in instruments of various kinds, including the transmission of laser beams and infrared and ultraviolet light where the crystals are used as domes or windows in missiles and related devices, communication transmission stations and the like. Doped crystals effective as scintillation phosphors are used for the detection of ionizing radiation in conjunction with a photomultiplier tube in devices ranging from simple scintillation counters to sophisticated camera plates for medical use in connection with the analysis of gamma radiation emanating from patients who are injected with specific active isotopes.
In the applications mentioned hereinbefore, as well as in many others, shaped articles transparent to the aforementioned wave length range have been limited in size by the peculiar physical properties of ionic crystals. The demanding requirements of windows, domes and lenses large enough for modern commercial and military requirements are antithetical to the well-known size limitation of shaped articles made from optically integral ionic crystals. Relatively large shaped articles have been prepared from hot-pressed powders of calcium fluoride and other alkali metal and alkaline earth metal halides. In addition to finely divided powders of polycrystalline materials in the range from about 20 to about 500 microns, single crystal fragments the average particle size of which ranged from less than 10 microns to several millimeters have also been used. The hot-pressed shaped articles formed have included lenses, windows and the like which are radiation-permeable, though with varying degrees of permeability depending upon the wavelength of radiation. Characteristically, these hotpressed articles are incapable of transmitting radiation coherently. The deficiency of hot-pressed crystals and powders, formed into articles as described in U.S. Pat. No. 3,359,066 are known to have markedly inferior properties as compared to lenses, domes and windows made from single crystals, or macrocrystal artificially melt-grown ingots. More refined hot-pressed polycrystalline materials, with random orientations of the grains, have been made as disclosed in U.S. Pat. No. 3,453,215. The method includes many purification steps of fine powder which is then hot-pressed into a laser host material. Despite the refinements, these materials are generally not fully dense, that is, they are less than 100 percent theoretical density and have an unacceptably high absorption coefficient for the transmittance of high-power laser beams in excess of about 1,000 watts/cm.sup.2.
Macrocrystal ingots are presently grown in sizes up to about 36 inches in diameter and about 1 foot long, wherein individual crystal grains may range in size up to a nominal diameter of about 8 inches or more. Large as these ingots are, they are not large enough; more important, were these ingots large enough, they would not have sufficient strength.
Thus, as of the present time, where a high quality, optically integral shaped article is required, whether it is for use as a scintillation phosphor, laser window, or an infrared or ultraviolet transmitting lens, when a relatively large shaped article is required, several individual single crystal pieces, or sections from a macrocrystal ingot are individually sawed off the melt-grown ingot and then adhesively bonded together to form the larger shaped composite. Notwithstanding the arduous devotion which is a perquisite of successful fabrication of a relatively large shaped article having superior radiation transmission properties, the end result was an article of determined fragility and transmission properties so marginally superior to those of hot-pressed articles as to negate the use of such relatively large composites for all but those applications where cost is not a factor. Moreover, a composite formed in this manner suffers from the drawbacks of degradation of light output due to the optical interfaces. No matter how carefully the faces of sections are polished before they are bonded into a composite, there is no way of eliminating the undesirable effects of the interface. Particularly in the case of windows for lasers, where it is essential that the radiation be maintained in coherent form, it is impractical to use a composite of macrocrystal sections. Assuming that adjacent macrocrystal sections were cleaved along matching planes, no matter how carefully the faces are polished, there would exist between the faces some macroscopic voids, however small, which would interrupt the transmission of coherent light. If the faces are adhesively bonded, the energy level of the laser to be transmitted would be predicated upon the temperature sensitivity of the energy-absorptive adhesive used. Assuming it is possible to maintain the adhesive at a relatively low temperature, there would still be the problem of matching the refractive index of the adhesive with that of the crystal. The slightest difference in refractive index would generate reflections that would destroy the usefulness of the composite as a laser window.
In pending applications Ser. Nos. 180,087; 179,787; 139,217; and 166,725 are described the formation of scintillation phosphors formed as extrudates of unrestricted length and arbitrary cross sections by extrusion of a single crystal or macro-crystal ingot at a temperature below its melting point and under sufficient pressure to form a coherent, homogeneous, fully dense polycrystalline material; stated differently, the shaped polycrystalline scintillation phosphors are formed by a process of extrusion or extruding, defined as: "to shape (as metal, plastic, rubber) by forcing through a specially designed opening often after a previous heating of the material or of the opening or of both" (see Webster's 3rd New International Dictionary, G. & C. Merriam Company, publishers, Springfield, Mass. 1966). In a more technical sense, extrusion is defined as "shaping (metal) into a chosen continuous form by forcing it through a die of appropriate shape" (see Definitions of Metallurgical Terms, p. 6, Metals Handbook, 1946 Edition, published by The American Society for Metals).
Extrusion of normally frangible macrocrystal ingots as disclosed in the aforementioned patent applications, and in the prior art, is effected in a confined zone and is distinct from press-forging as claimed in this invention. For example, it is known that a single crystal billet of sodium chloride may be fitted tightly into an extrusion chamber and forced through an extrusion dye maintained at various temperatures above 300.degree.C. to yield a rod of polycrystalline sodium chloride which is completely clear and free from porosity and therefore, optically integral. (See "Mechanical Properties of Polycrystalline Sodium Chloride" by R. J. Stokes, Proceedings of the British Ceramic Society, Vol. 6, p. 192, June, 1966). This work was done in connection with a study of the mechanical properties of polycrystalline sodium chloride in relation to those of polycrystalline magnesium oxide which has a similar lattice structure but a melting point so high (2,650.degree.C.) as to make the direct study of the "more technologically significant material," namely magnesium oxide, all but impossible. This study is not directly related to the press-forged crystals of this invention.
It is also known that an almost completely transparent polycrystalline cesium bromide disk can be made under pressure in a steel mold from a single crystal pressed at 8,000 PSI at 300.degree.C. under vacuum, heated to 400.degree.C. and then cooled to 100.degree.C. while being maintained under pressure. The single crystal was maintained in a confined mold, under radial constraint, presumably to ensure the formation of a disk without fractures. The reported study entitled "The Mechanical Behavior of Single Crystal and Polycrystalline Cesium Bromide" by L. D. Johnson and J. A. Pask, J. Am. Ceram. Soc., 47, 9, 437-444, 1964, was undertaken because CsBr has an interpenetrating simple cubic structure rather than an interpenetrating face-centered cubic structure characteristic of rock salt or lithium fluoride. The authors reported that single crystals of cesium bromide were found to be "soft" and ductile if they were favorably oriented relative to the loading axis to activate the 110 slip systems. There is no indication that any deformation of the single crystal was effected. Neither is there any indication that if there was any deformation, that the deformation of the crystal was effected without fracture, or, whether any fracture that occurred was reconstituted or `healed` under the particular conditions of temperature, pressure and constraint. Having formed an almost completely transparent polycrystalline disk from a monocrystalline disk in a confined mold, the authors studied the polycrystalline structure so formed and stated "Because of the lack of 5 independent slip systems for this crystal structure, general plastic deformation of polycrystalline cesium bromide cannot occur without formation of voids or gaps between the grains unless some other deformation mechanism can operate. Kinking, the only other possible mechanism observed, required considerably higher stress levels than were attained in these specimens" (bottom of p. 443). It is hypothesized that the lack of radial constraint in the press-forged crystals of this invention provides the basis for the `some other deformation mechanism` which permits total optical integrity and transparency, rather than almost complete transparency, whether the starting material is an essentially single macrocrystal ingot or a polycrystalline mass. This unexpected transparency or unhindered permeability extends not only to light in the infra-red, visible and ultra-violet regions where it is greatly desirable, but to high-energy coherent laser beams in excess of 1,000 watts/cm.sup.2 where it is a necessity.